Fibroblast activation protein (FAP) is a serine protease expressed on the surface of tumor-associated fibroblasts (TAFs) in epithelial cancers and expressed to a lesser extent in healthy tissues. FAP appears to promote tumor growth, and FAP+ TAFs appear to suppress tumor killing by effector T cells (Teff). However, the role of FAP proteolytic activity in these effects is not fully understood. For instance, is FAP proteolytic activity involved in immunosuppression by FAP+ TAFs? Is FAP proteolytic activity involved in tumor promotion by non- immune effects? Indeed, it is possible that FAP promotes tumor growth by several mechanisms. Mechanistic knowledge of FAP is needed for evaluation of its potential as a target for drug discovery. Understanding of the possible immunoregulatory functions of FAP is particularly important. Cancer vaccines can prime T cells against tumor antigens; but the immune responses seldom produce durable tumor regression clinically. Immunosuppression of T cells by FAP+ TAFs at the tumor site seems likely to contribute to the failure of cancer vaccines. Consequently, pharmacological agents that inhibit or ablate FAP+ TAFs could relieve immunosuppression and act as adjuvants that enable cancer vaccines to achieve clinical efficacy. To date, the unavailability of pharmacological inhibitors that target FA with selectivity over related serine proteases has hindered investigation of FAP proteolytic activity in cancer. Non-specific inhibitors have yielded results that implicate serine proteases in tumor growth and immune regulation; but the relevant target in vivo remains elusive. Bachovchin et al recently discovered how to make FAP-selective inhibitors and cytotoxic pro-drugs that are targeted by FAP to the tumor site. The agents will be used to probe FAP and FAP+ TAF function in models of epithelial cancer in which FAP is known to promote tumor growth. Investigation of two different pharmacological approaches - FAP inhibition and cytotoxic ablation of FAP+ TAFs - will reduce risk of failure in the early stages of the project. The Specifi Aims include: comparison of antitumor effects achieved by inhibition of FAP activity versus pharmacological ablation of FAP+ TAFs, characterization of the possible immune or non-immune mechanisms of action, and evaluation of the feasibility of FAP-targeted anticancer agents by determining therapeutic index and characterizing mechanism-based toxicities that might arise due to expression of FAP in some normal tissues. It will be important to evaluate any hematopoietic toxicity of FAP- targeted agents that might arise from interactions with FAP-expressing stromal fibroblasts in bone marrow. The proposed research will address basic questions about the functions of FAP in tumor progression and support the long-term goal of developing a FAP-targeted pharmacological approach to cancer treatment.